Added hooks to to enrich the raw swagger data

I pushed PR #47 a while ago, but wasn't sure about the solution. After thinking about it again, I came to the conclusion it wasn't a very nice solution.

This PR introduces the concept of enriching the swagger data. You could implement a class which implements <code>ISwaggerDataEnricher</code>. This interface exposes four methods, which are called by the <code>SwaggerAnnotationsConverter</code> just after an instance containing data was created.

I implemented the <code>JsonNetEnricher</code> as an example, which uses reflection to find <code>JsonObjectAttribute</code> and <code>JsonPropertyAttribute</code> by name (so no dependency to Newtonsoft.Json is needed). It uses the data from this properties to enrich the <code>SwaggerModelData</code> and <code>SwaggerModelPropertyData</code>.

An approval test was added which demonstrates the resulting output.

I added this post-processing only in the Nancy.Swagger.Annotation project, because I'm not sure if this approach is suitable/needed when working with the <code>DefaultSwaggerMetadataConverter</code>.